Femoral head assembly with variable offset

ABSTRACT

A proximal femoral ball assembly having a variable offset that is selectively adjustable to conform to various anatomical conditions encountered during a femoral surgical procedure. The femoral ball assembly generally includes a head, a neck, and an adjustment mechanism. The head has a smooth spherical outer surface that is adapted to engage an acetabular component or native acetabulum. The neck extends outward from the head and removeably connects to the head using a threaded attachment.

CROSS-REFERENCE TO RELATED APPLICATIONS

This continuing patent application claims priority under 35 U.S.C. §120to co-pending nonprovisional patent application Ser. No. 10/613,334,filed on Jul. 3, 2003, the disclosure of which is expressly incorporatedby reference herein.

BACKGROUND

1. Field Of The Invention

The disclosure generally relates to implantable orthopedic prosthesesfor total hip arthroplasty and, more particularly, to a proximal femoralhead assembly having a variable offset that is selectively adjustable toconform to various anatomical conditions encountered during a femoralsurgical procedure.

2. Background Of The Invention

During a total hip arthroplasty, a femoral stem is implanted into theintramedullary canal of a femur. After the stem is inserted to theproper depth and orientation, a femoral head or ball is attached to theproximal end of the stem. This head fits into the socket of anacetabular component and provides a joint motion surface forarticulation between the femoral prosthesis and acetabulum. A neck ortrunion extends between the femoral ball and stem. In many embodiments,this neck generally has a cylindrical configuration with one endconnected to the ball and one end connected to the stem.

Several critical features are important to ensure that the femoral hipprosthesis properly functions once implanted in the femur. One of thesefeatures is the femoral head “offset.” Femoral offset is the horizontaldistance from the center of rotation of the femoral head to a linebisecting the long axis of the femur from a standing A-P x-ray.Similarly, the offset of the proximal femoral component of a hipprosthesis is the horizontal distance from the center of rotation of thefemoral head to the long axis of the stem.

One important decision that must be made during hip surgery is how muchoffset should occur between the femoral ball and stem. If the offsetdoes not match the natural anatomical needs of the patient, then theprosthesis can be positioned too far laterally or medially. Anunnecessary decrease in offset greatly affects the success and properfunction of the hip implant after surgery.

A decrease in femoral offset medially shifts or moves the femur closerto the pelvis. This decrease can result in impingement of the prosthesisfor some patients after surgery. A medial shift can also cause softtissue surrounding the implant to become loose or lax. Impingement andsoft tissue laxity can further lead to instability of the implant,subluxation, and even dislocation. As a further disadvantage, when theoffset decreases, the abductor muscles utilize a greater force tobalance the pelvis. This increase in force creates a discrepancy thatmay result in a limp for the patient. As another consequence, theresultant force across the hip joint also increases, and this increasecan lead to greater polyethylene wear between the femoral ball andprosthetic acetabular component.

In contrast to a decreased offset, an increase in femoral offsetlaterally shifts or moves the femur farther from the pelvis. In someinstances, an increase in offset is desirable. This increase can reducethe risk of impingement and improve soft tissue tension, resulting in amore stable implant. Further, the adductor muscles can be more properlybalanced and improve the gait of the patient. Further, proper balanceand alignment can lead to less wear and loosening over time.

Manufacturers and designers of femoral hip prosthesis recognize theshortcomings associated with decreased offset and endeavor to match theoffset with the anatomical needs of the patient. In order to remedythese shortcomings, femoral hip prostheses are sold with differentoffsets. The number and degree of different offsets vary between themanufacturers. A typical prosthetic system can include three to fivedifferent offsets for each femoral ball size. For example, amanufacturer may provide femoral balls with offsets of −4 mm, 0 mm, +4mm, +8 mm, and +12 mm. These offsets would be available for five or sixdifferent ball sizes. In short, the manufacturer is required to have aninventory of 18 to 30 different femoral heads.

An inventory of femoral heads of this magnitude is enormous. Further,the costs associated with maintaining and distributing this inventoryare very great for a company. This large inventory, then, is a cleardisadvantage.

As another important disadvantage, manufacturers offer the femoral headoffsets in fixed, discrete, large increments. As noted, the offsets, forexample, may be offered in increments of −4 mm, such as offsets of −4mm, 0 mm, +4 mm, +8 mm, and +12 mm. These fixed increments though maynot exactly match the anatomical offset that the patient needs. Forexample, if the patient requires an offset of +6 mm, then the surgeonmust choose between an offset of either +4 mm or +8 mm.

It therefore would be advantageous to provide a proximal femoral headhaving a variable offset that is selectively adjustable to conform tovarious anatomical conditions encountered during a femoral surgicalprocedure.

SUMMARY OF THE INVENTION

The present invention is directed to implantable orthopedic prosthesesfor total hip arthroplasty and, more particularly, to a proximal femoralball assembly having a variable offset that is selectively adjustable toconform to various anatomical conditions encountered during a femoralsurgical procedure.

The femoral ball assembly generally comprises a head, a neck, and anadjustment mechanism. The head has a smooth spherical outer surface thatis adapted to engage an acetabular component or native acetabulum. Theneck extends outward from the head and removeably connects to the headusing a threaded attachment.

The adjustment mechanism provides a variable offset for the femoral ballassembly. More specifically, the adjustment mechanism varies the lengththat the neck protrudes from the head. As this length increases, thefemoral offset correspondingly increases. As this length decreases, thefemoral offset correspondingly decreases. One important advantage thenis that the surgeon can intra-operatively select from a wide array offemoral offsets. These offsets can be provided with a small number ofcomponents. As such, a large, expensive inventory of differently sizedfemoral balls with different offsets is not necessary.

Another advantage of the present invention is that a plurality offemoral offsets can be offered in small increments. The offsets, forexample, can be offered in 1 mm increments. These small increments canbe used to more closely match the natural anatomical needs of thepatient. Further, these offsets can be offered in a range from about −10mm to about +10 mm, but a range of up to about +30 mm is within thescope of the invention.

In another embodiment, a femoral ball system is provided. The system hasa plurality of differently sized femoral heads and spacers. These headsand spacers can be utilized with a single neck to provide a multitude offemoral offsets with a plurality of differently sized spherical heads orballs.

In yet another embodiment, two separate axes extend through the femoralball assembly. A first axis or central axis is concentric with the bodyof the spherical head, and a second axis or eccentric axis is concentricwith the threaded bore of the head. This second axis is also concentricwith the adjustment mechanism and neck of the femoral ball assembly.These two axes are parallel to each other and form an acute angle withthe longitudinal axis of the stem.

In one form thereof, the present invention provides an assembly,including a femoral head assembly connectable to a femoral hip stem, theassembly including: a femoral head having a body with a spherical outersurface adapted to articulate within an acetabular component, the bodyhaving a threaded bore; a plurality of spacers of varying thickness, atleast one of the plurality of spacers adapted to be inserted into thethreaded bore; a first neck having an externally threaded portion and atleast one of an external tapered portion and an internal bore defining ainternal tapered portion, the externally threaded portion being adaptedto be threadably engaged with the threaded bore of the body of thefemoral head; wherein the first neck is adapted to extend outwardly fromthe femoral head in various lengths, each length corresponding to thethickness of the at least one of the plurality of spacers inserted intothe threaded bore; and a femoral hip stem, the femoral hip stem having asecond neck including at least one of an external tapered surface and atapered internal bore, the one of the external tapered surface and thetapered internal bore of the femoral hip stem sized to engage the one ofthe external tapered portion and the internal bore of the first neck toform a Morse taper connection between the femoral hip stem and the firstneck.

In another form thereof, the present invention provides an assemblyincluding: a femoral head having a body with an outer surface adapted toarticulate with an acetabular component, the femoral head comprising aninternally threaded bore; a first neck having a first externallythreaded end adapted to be threadingly connected to the internalthreaded bore of the femoral head and a second end comprising at leastone of an external tapered portion and an internal tapered bore; afemoral hip stem, the femoral hip stem comprising a second neck havingat least one of an external tapered surface and a tapered internal bore,the at least one of the external tapered surface and the taperedinternal bore of the femoral hip stem sized to engage the at least oneof the external tapered portion and the internal tapered bore of thefirst neck to form a Morse taper connection between the femoral hip stemand the first neck; and at least one spacer adapted to be positionedwithin the internally threaded bore of the femoral head between thefirst end of the first neck and the femoral head, wherein the at leastone spacer engages the first end of the first neck and a bottom surfaceof the internally threaded bore of the femoral head when the first neckis threadingly coupled and seated in the internally threaded bore of thefemoral head, the first neck extending outwardly from the femoral headby a length that corresponds to a thickness of the at least one spacerpositioned within the internally threaded bore.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view of a femoral hip stem, an assembled femoralball assembly according to the invention, and an acetabular component.

FIG. 2 is an exploded view of the femoral ball assembly according to theinvention.

FIG. 3 is an exploded view of an alternate embodiment of the femoralball assembly.

FIG. 4 is an exploded view of an alternate embodiment of the femoralball assembly.

FIG. 5 is an exploded view of a femoral ball system according to theinvention.

FIG. 6 is an exploded view of an alternate embodiment of the femoralball assembly.

FIG. 7 is an exploded view of another embodiment of the femoral ballassembly.

FIG. 8 is an exploded view of yet another embodiment of the femoral ballassembly.

FIG. 9 is an enlarged perspective view of the assembled femoral ballassembly of FIG. 8.

FIG. 10 is an exploded view of another embodiment of the femoral ballassembly.

DETAILED DESCRIPTION

Looking to FIG. 1, an implantable orthopedic femoral hip stem or implant10, a proximal femoral head or ball assembly 12 according to theinvention, and an acetabular component 14 are shown. These componentsare connectable together for use in a total hip arthroplasty.

Stem 10 includes a body 20 that extends from a proximal region 22 to adistal region 24. A longitudinal or long axis 25 extends through thebody. The body tapers downwardly and generally has a cylindrical ortrapezoidal shape with the distal end being rounded to facilitateinsertion into the intramedullary canal of a femur. The proximal region22 includes a proximal body portion or trochanteral portion 26 having acylindrical bore 28, a collar 30, and a top surface 32. A neck 34extends outwardly from the top surface 32. The neck 34 has a taperedbody that connects to the femoral ball assembly 12.

The acetabular component 14 is configured to fit in the acetabulum of apatient and is formed from an outer shell 40 and an inner liner orbearing component 42. The shell is generally shaped as a hemisphericalcup defined by an outer hemispherical surface or bone engaging surfaceand an inner hemispherical surface connected to the bearing component.The outer surface can be porous or textured while the inner surface issmooth and adapted to articulate with the femoral ball assembly 12.

One skilled in the art will appreciate that the femoral ball assembly ofthe present invention can be employed with various implants and implantdesigns without departing from the scope of the invention. The stem 10,for example, can be the Apollo™ Hip or Natural™ Hip manufactured byCenterpulse Orthopedics Inc. of Austin, Tex.; and the acetabularcomponent 14 can be the Allofit™ or Converge™ acetabular systemmanufactured by the same company.

Looking now to FIGS. 1 and 2, the femoral ball assembly 12 is adapted toconnect at one end to the femoral hip stem 10 and at another end to theacetabular component 14. The femoral ball assembly 12 comprises a head50, a neck 52, and an adjustment mechanism 54. A central axis 56 extendsthrough the center of the head, the neck, and adjustment mechanism.

Head 50 has a body that is shaped as a partial sphere. This body has asmooth outer surface 60 adapted to engage and slideably articulate withthe bearing component 42 of the acetabular component 14. A collar ortapered transition 62 circumferentially extends around a base 64 of thespherical body. A threaded and cylindrical bore 66 extends into thebody.

Neck 52 has a generally straight cylindrically shaped body that extendsfrom a first end 70 to a second end 72. The first end 70 includes a bore74 adapted to receive and engage neck 34 of stem 10. Specifically, bore74 has a tapered cylindrical shape with smooth inner walls. This taperis adapted to form a Morse taper connection with neck 34 when stem 10and femoral ball assembly 12 are connected together. Preferably, bore 74does not extend completely through neck 52 but stops at a generallyplanar end surface 76 shown as dashed lines inside the body. The secondend 72 includes an externally threaded section 78. This threaded sectionis adapted to threadably engage with threaded bore 66 of head 50.

Adjustment mechanism 54 is adapted to vary the effective length “L”(shown in FIG. 1) of neck 52 extending outwardly from head 50. Effectivelength “L” extends from first end 70 to base 64 of the body of the head50. Adjustment mechanism 54 is further adapted to provide a variable“offset” between the femoral head assembly 12 and femoral stem 10.Femoral offset is the horizontal distance from the center of rotation ofthe femoral head to a line bisecting the long axis 25 of the femur froma standing A-P x-ray. Similarly, the offset of the proximal femoralcomponent of a hip prosthesis is the horizontal distance from the centerof rotation of the femoral head to the long axis of the stem.

As shown in FIG. 2, adjustment mechanism 54 includes a spacer 80. Thisspacer has a short cylindrical shape and is preferably formed as a solidround or coin-shape. Spacer 80 is adapted to be removeably positioned inbore 66 of head 50.

An adjustment mechanism of the present invention may be used in variousways to provide a variable offset between the femoral head and femoralstem. FIG. 2 shows one example. While spacer 80 is removed from the bore66, neck 52 can be threadably connected to head 50. Once the neck isfully seated and threaded into bore 66, the neck 52 will have aneffective length L1. This length equals the distance from first end 70to base 64 of head 50. Neck 52 can be removed from head 50 and spacer 80then placed inside of bore 66. Spacer 80 has a thickness equal to T.With the spacer 80 placed in bore 66, neck 52 can be threadablyconnected to head 50. Once the neck is fully seated and threaded intobore 66, the neck 52 will have an effective length L2, wherein L2=L1+T.In other words, the spacer lengthens the distance from first end 70 tobase 64 an amount equal to its thickness T.

The embodiment in FIG. 2 is able to provide a femoral ball with twodifferent offsets between the femoral head and femoral stem. The lengthdifference between these two offsets depends on the thickness of thespacer. The spacer can be sized to have various thicknesses. As such,the offset can be varied by varying the thickness of the spacer.Further, a plurality of spacers with different thicknesses can beprovided. For example, these spacers could have thicknesses with onemillimeter increments, such as 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, etc.Other increments, of course, are within the scope of the invention.

FIGS. 1 and 2 show a Morse taper connection between the neck 34 of stem10 and neck 52 of femoral ball 12. In this connection, neck 52 functionsas the female component and neck 34 functions as the male component. Oneskilled in the art will appreciate that this connection can be alteredand still within the scope of the invention. For example, neck 34 can beformed with a bore to function as the female component, and neck 52 canbe formed as a solid tapered cylinder to function as the male component.Various types of other connections could also be employed to connect thestem to femoral ball assembly and still remain within the scope of theinvention. These connections include, but are not limited to, press-fitconnections, locking rings, radial or expandable devices (such assleeves or collars), nitinol or other superelastic materials, taperconnections, locking connections, various polygonal connections (such astriangular, square, hexagonal, or trapezoidal), and the like. In short,various ways can be used to connect the femoral ball 12 to the stem 10.

Looking now to FIG. 3, an alternate femoral ball assembly 100 is shownand includes a head 102, a neck 104, and an adjustment mechanism 106.Head 102 and neck 104 are identical to head 50 and neck 52 described inconnection with FIGS. 1 and 2. In FIG. 3, adjustment mechanism 106includes two spacers 110 and 112. Preferably, these spacers havedifferent thicknesses. These thicknesses, for example, can be selectedfrom a group with one or two millimeter increments, such as 1 mm, 2 mm,3 mm, 4 mm, 5 mm, 6 mm, etc.

Looking now to FIG. 4, an alternate femoral ball assembly 120 is shownand includes a head 122, a neck 124, and an adjustment mechanism 126.Head 122 and neck 124 are identical to head 50 and neck 52 described inconnection with FIGS. 1 and 2. In FIG. 3, adjustment mechanism 126includes three spacers 130, 132, and 134. Preferably, these spacers havedifferent thicknesses. These thicknesses, for example, can be selectedfrom a group with one or two millimeter increments, such as 1 mm, 2 mm,3 mm, 4 mm, 5 mm, 6 mm, etc.

FIGS. 3 and 4 offer a multitude of different offsets between the femoralhead and femoral stem. These offsets depend on the number and thicknessof spacers used. Further, these spacers can be used alone (i.e., onespacer placed inside the bore of the femoral head) or used inconjunction with other spacers. In the latter scenario, two, three,four, or more spacers can be stacked on top of each other and thenplaced in the bore of the femoral head. This stacking arrangement canprovide a wide range of offsets in small increments. Looking to FIG. 4to illustrate an example, spacer 130 can have a thickness of 1 mm;spacer 132 can have a thickness of 2 mm; and spacer 134 can have athickness of 4 mm. These spacers could be used, alone or in stackedcombinations with each other, to have thicknesses of 0 mm, 1 mm, 2 mm, 3mm, 4 mm, 5 mm, 6 mm, or 7 mm. Thus, these three spacers can provide afemoral ball with 8 different offsets, assuming one offset (0 mm) usesno spacer at all.

One advantage of the present invention is that the number and thicknessof spacers can vary to provide a multitude of offsets between thefemoral head and femoral stem. As another example, four spacers could beprovided to have thicknesses of 1 mm, 1 mm, 3 mm, and 6 mm. These fourspacers would allow twelve different offset options (0 mm, 1 mm, 2 mm, 3mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, and 11 mm). This exampleis illustrated in FIG. 5.

FIG. 5 shows a femoral ball system 150. System 150 includes a singleneck 152, a plurality of femoral heads 154A, 154B, and 154C, and anadjustment mechanism 156 having a plurality of spacers 158A, 158B, 158C,and 15 8D. The neck, heads, and spacers are generally identical to theneck, head, and spacer discussed in connection with FIGS. 1 and 2. Asone difference, the heads and spacers are offered in different sizes.Preferably, each head is sized differently, such as small, medium, andlarge. More specifically, a plurality of heads could be provided to haveouter diameters of 22 mm, 26 mm, 28 mm, 32 mm, 38 mm, and 44 mm.Further, the spacers can be sized to maximize the number of differentoffsets while reducing the overall number of inventory components. Onesuch size combination is spacers having thicknesses of 1 mm, 1 mm, 3 mm,and 6 mm. One skilled in the art will appreciate that many variations inthe number and size of heads and spacers are within the scope of theinvention.

FIG. 6 shows another embodiment of a femoral ball assembly 200 thatincludes a head 202, a neck 204, and an adjustment mechanism 206. Head202 and neck 204 are generally similar to head 50 and neck 52 describedin connection with FIGS. 1 and 2. In FIG. 3, adjustment mechanism 206includes a biasing member 210 that is adapted to be placed into thethreaded bore 212 of head 202. Biasing member 210 is shown as a coiledspring, but one skilled in the art will appreciate that many types ofbiasing members are also available.

During use, the biasing member 210 is placed in bore 212, and then neck204 is threadably engaged with head 202. As the neck screws into thebore, the biasing member provides a greater and greater force againstthe neck. In turn, the torque required to screw the neck increases as itthreads into the bore. In one embodiment, this torque can be calibratedto specific offset values. In other words, specific torque values cancorrespond to specific offsets. In another embodiment, indicia or aplurality of calibration marks 220 can be placed on the surface of theneck. Preferably these marks correspond to distinct, finite offsets.Markings could be given to illustrate five different offsets in 4 mmincrements, such as −4 mm, 0 mm, +4 mm, +8 mm, and +12 mm. One skilledin the art will appreciate that various indicia can be used toillustrate various offsets.

FIG. 7 shows another embodiment of a femoral ball assembly 300 thatincludes a head 302, a neck 304, and an adjustment mechanism 306. Neck304 and adjustment mechanism 306 are identical to the head andadjustment mechanism discussed in connection with FIGS. 1-4. Head 302 issimilar to the head 50 discussed in connection with FIGS. 1 and 2 withone important difference: Head 302 is eccentric. More specifically, thebody of head 302 has a central axis 310 that passes through the centerof the body. The body also has a second or eccentric axis 312 that isparallel to the central axis and passes through the center of threadedbore 314. The center of bore 314 is thus offset or eccentric with thecentral axis 310 of the body. As shown, the center of spacers 320 and322 and neck 304 are concentric with bore 314.

The embodiment in FIG. 7 is advantageous because the neck 304 iseccentric or offset from the head 302. This eccentric neck provides anincrease range of motion once connected to the femoral hip stem 10 (FIG.1). This increase in range of motion more fully emulates the anatomicalmovements of a natural hip. Additionally, this increase in range ofmotion provides more joint stability to the implanted prosthesis. As yetanother advantage, the eccentric neck provides a femoral hip prosthesisthat is less likely to experience impingement, subluxation, or evendislocation.

FIGS. 8 and 9 show another embodiment of a femoral ball assembly 400that includes a head 402, a neck 404, and an adjustment mechanism 406.Head 402 is identical to the head 50 discussed in connection with FIGS.1 and 2. Neck 404 is similar to the neck 52 discussed in connection withFIGS. 1 and 2 with one important difference: Neck 404 includes a collaror shoulder 407 at first end 408. This collar has a circular orring-shape and extends outwardly from an outer surface 410 of neck 404.

The adjustment mechanism 406 accomplishes a similar function to theadjustment mechanism 54 discussed in connection with FIGS. 1 and 2, butthe function is performed in a different way. More specifically, theadjustment mechanism 406 includes a plurality of spacers 412 and 414.These spacers have a ring-shape or C-clip shape. Unlike spacer 80 inFIGS. 1 and 2, spacers 412 and 414 are not adapted to fit inside bore420 of head 402. Instead, spacers 412 and 414 are adapted to fit aroundthe second end 424 of neck 404. As best shown in FIG. 9, the spacersextend through second end until they abut against collar 407.

Adjustment mechanism 406 is adapted to vary the effective length of neck404. As discussed in connection with FIGS. 1 and 2, the adjustmentmechanism is further adapted to provide a variable offset between thefemoral head and femoral stem. As shown in FIG. 9, the effective lengthof the neck is increased by a distance “D” equal to the thickness ofspacer 412 plus the thickness of spacer 414. These spacers may haveequal thicknesses or unequal thicknesses.

FIGS. 8 and 9 show the spacers with a C-clip shape. One skilled in theart will appreciate that other configurations are within the scope ofthe invention. By way of example, these configurations include a fullring-shape or retaining ring shape.

FIG. 10 shows another embodiment of a femoral ball assembly 500 thatincludes a head 502 and a neck 504. The head 502 and neck 504 aresimilarly configured to the head 50 and neck 52 of FIGS. 1 and 2 withseveral important differences. First, femoral ball assembly 500 does notinclude a separate adjustment mechanism. Bore 506 has an internallythreaded wall that extends along the entire depth of the bore. The neckcan thread along the entire length of the bore. Thus, the length of theneck can be varied a distance “D” approximately equal to the depth ofthe bore. Further, indicia or a plurality of calibration marks 510 canbe placed on the outer surface of the neck. Preferably these markscorrespond to distinct, finite offsets that the surgeon can view andread during a surgical procedure. Markings could be given to illustratefive different offsets in 4 mm increments, such as −4 mm, 0 mm, +4 mm,+8 mm, and +12 mm. One skilled in the art will appreciate that variousindicia can be used to illustrate various offsets and increments.

In order to prevent, the neck from loosening once a desired offset ischosen, a locking mechanism can be used to prevent relative rotationalmotion between the neck and head.

The femoral head assembly of the present invention may be manufacturedof a wide array of biocompatible materials that are known in the art.These materials include ceramics, stainless steel, titanium, and cobaltchrome alloys. Further, the adjustment mechanism may be manufacturedfrom a broader range of materials, such as various elastomers known inthe art. Preferably, such an elastomer has a well defined, controlled,and reproducible Poisson's ratio that can be used to adjust and monitorthe femoral offset by tightening or loosening the neck to a given loador torque level.

FIGS. 1-9 illustrate a femoral ball assembly wherein the head, neck, andadjustment mechanism are separate components that are removeablyconnectable to each other. Other embodiments are within the scope of theinvention. For example, the adjustment mechanism could be permanentlyconnected to the neck or head. Further, the components can be configuredto not be removeable from each other once they are connected. Furtheryet, the invention can utilize various locking mechanisms to keep ormaintain the neck within the head and prevent any unintentionalloosening.

Although illustrative embodiments have been shown and described, a widerange of modifications, changes, and substitutions is contemplated inthe foregoing disclosure; and some features of the embodiments may beemployed without a corresponding use of other features. Accordingly, itis appropriate that the appended claims be construed broadly and in amanner consistent with the scope of the embodiments disclosed herein.

1. An assembly, comprising a femoral head assembly connectable to afemoral hip stem, the assembly comprising: a femoral head having a bodywith a spherical outer surface adapted to articulate within anacetabular component, said body having a threaded bore; a plurality ofspacers of varying thickness, at least one of said plurality of spacersadapted to be inserted into the threaded bore; a first neck having anexternally threaded portion and at least one of an external taperedportion and an internal bore defining a internal tapered portion, saidexternally threaded portion being adapted to be threadably engaged withsaid threaded bore of said body of said femoral head; wherein said firstneck is adapted to extend outwardly from said femoral head in variouslengths, each length corresponding to the thickness of said at least oneof said plurality of spacers inserted into the threaded bore; and afemoral hip stem, said femoral hip stem having a second neck includingat least one of an external tapered surface and a tapered internal bore,said one of said external tapered surface and said tapered internal boreof said femoral hip stem sized to engage said one of said externaltapered portion and said internal bore of said first neck to form aMorse taper connection between said femoral hip stem and said firstneck.
 2. The assembly of claim 1, further comprising a plurality offemoral heads, each of said plurality of femoral heads having adifferent outer diameter.
 3. The assembly of claim 1, further comprisinga plurality of femoral heads, each of said plurality of femoral headsdefining a central axis extending through a center of said body, atleast one of said plurality of femoral heads further comprising aneccentric axis extending through a center of said threaded bore, whereinsaid eccentric axis is parallel to and offset from said central axis. 4.The assembly of claim 1, wherein said thicknesses of said plurality ofspacers are provided in 1 millimeter increments.
 5. The assembly ofclaim 1, wherein said plurality of spacers have at least three differentthicknesses.
 6. The assembly of claim 1, wherein said threaded bore ofsaid femoral head is sized to receive more than one of said plurality ofspacers to vary an offset of said first neck from said femoral head. 7.An assembly comprising: a femoral head having a body with an outersurface adapted to articulate with an acetabular component, said femoralhead comprising an internally threaded bore; a first neck having a firstexternally threaded end adapted to be threadingly connected to theinternal threaded bore of said femoral head and a second end comprisingat least one of an external tapered portion and an internal taperedbore; a femoral hip stem, said femoral hip stem comprising a second neckhaving at least one of an external tapered surface and a taperedinternal bore, said at least one of said external tapered surface andsaid tapered internal bore of said femoral hip stem sized to engage saidat least one of said external tapered portion and said internal taperedbore of said first neck to form a Morse taper connection between saidfemoral hip stem and said first neck; and at least one spacer adapted tobe positioned within said internally threaded bore of said femoral headbetween said first end of said first neck and said femoral head, whereinsaid at least one spacer engages said first end of said first neck and abottom surface of said internally threaded bore of said femoral headwhen said first neck is threadingly coupled and seated in saidinternally threaded bore of said femoral head, said first neck extendingoutwardly from said femoral head by a length that corresponds to athickness of said at least one spacer positioned within said internallythreaded bore.
 8. The assembly of claim 7, wherein said at least onespacer comprises a plurality of spacers and wherein at least one of saidplurality of spacers engages said first end of said first neck andanother of said plurality of spacers engages said bottom surface of saidinternally threaded bore, and wherein said first neck extends outwardlyfrom said femoral head by a length that corresponds to a combinedthickness of said plurality of spacers positioned within said internallythreaded bore.
 9. The assembly of claim 7, further comprising aplurality of femoral heads, each of said plurality of femoral headshaving a different outer diameter.
 10. The assembly of claim 7, furthercomprising a plurality of femoral heads, each of said plurality offemoral heads defining a central axis extending through a center of saidbody, at least one of said plurality of femoral heads further comprisingan eccentric axis extending through a center of said threaded bore,wherein said eccentric axis is parallel to and offset from said centralaxis.
 11. The assembly of claim 7, wherein said at least one spaceradjusts a femoral offset of said femoral head with respect to saidfemoral hip stem.
 12. The assembly of claim 7, wherein said at least onespacer has a thickness selected from the group consisting of 1millimeter, 2 millimeters, 3 millimeters, and 4 millimeters.
 13. Theassembly of claim 7, wherein said at least one spacer comprises at leastthree spacers, each of said at least three spacers having differentthicknesses.